Prototype of FlowGraph

Early prototype for discussing the way forward.
Before we start with the DSL sugar I would like that we express the functionality in terms of something similar to the raw addEdge methods.
yes, and another example that I would like to support:

    in1 ~> merge ~> out
    in2 ~> merge ~> out

i.e. injecting the identity flows to the graph representation, and then when materializing these identity flows can be shortcutted